Determining a width and/or thickness of a generally rectangular object

ABSTRACT

At least two cameras (10,12) view two points (A,B) displaced by a known distance on an article (S). Using a known relationship between two points (A,B) and the known position of cameras (10,12) in space the location of the viewed points (A,B) in space are determined by calculation. Similarly, the coordinates of other points on to article (S) are determined and hence dimensions such as width are determined.

FIELD OF THE INVENTION

This invention relates to a method and apparatus for determining adimension of an article such as the width or thickness of a plate orslab produced in a steel mill.

SUMMARY OF THE INVENTION

The invention may be said to reside in a method of determining adimension of an article, the method comprising the steps of viewing twopoints on the article with a sensing device, the two points beingdisplaced by a known relationship, using the known relationship and thetwo viewed points to determine the location in space of one of thepoints, and using the absolute location in space of another pointdetermining the dimension of the article between the one point and theanother point.

The invention may also be said to reside in an apparatus for determiningthe dimension of an article, the apparatus comprising two sensing deviceeach sensing one of two points on the article which are displaced by aknown relationship, a processing device for resolving the location inspace of one of the two points and for determining the distance betweenone point and another point to enable a dimension of the article to bedetermined.

The another point could be a known fixed point. However according to thepreferred embodiments of the invention, the another point is determinedby viewing a second pair of points including the another point, thesecond pair of points being displaced by a further known relationshipand determining the location in space of the another point from theviewed second pair of points and the further known relationship.

Preferably two sensing device each view one of the first two points anda further two sensing device each view one of the second two points.However only two sensing device could be used with one sensing deviceviewing one of the first two points and one of the second two points andthe other sensing device viewing the other of the first two points andthe other of the second two points. In one embodiment of the inventionthe two pair of points, which are displaced by a known distance, cancomprise points at the edges of a generally rectangular cross-sectionedarticle, and in a second embodiment, the pairs of points can be pointson an arc of a circle of a curved portion of the article.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiment of the invention will be described with referenceto the accompanying drawings in which:

FIG. 1 is a view of the first embodiment of the invention;

FIG. 2 is a view of a second embodiment of the invention;

FIG. 3 shows a third embodiment; and

FIG. 4 shows one preferred manner of making calculations according tothe preferred embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to FIG. 1 a system is shown for determining the width Wof a plate or slab of a generally rectangular cross-section. The systemcomprises a first sensor means comprising a first linescan camera 10 anda second linescan camera 12. Located at the other end of the plate orslab S is a second pair of linescan cameras (which are not shown). Thelinescan camera 10 views the point A at the top edge of the slab S andthe linescan camera 12 views the point B at the bottom edge of the slabS. The slab S could be hot enough to provide illumination for thecameras 10 and 12 or a back light (not shown) could be utilized.

The points A and B are displaced by the known thickness T of the slab Sand are assumed to be vertically displaced, one above the other. Sincethe thickness T of the slab S is known and the points A and B arevertically displaced, a computing apparatus 5 coupled to the linescancameras 10 and 12 can resolve the location of point A in space from theimages in the cameras. The other pair of linescan cameras (not shown) atthe other end of the slab S resolve the location of a correspondingpoint A at the top edge of the slab in a similar manner. Since thelocation in space of two points at the edges of the slab S are known,the distance between the two points can be determined by simple geometryto enable the width W of the slab to be determined.

A second embodiment of the invention is shown in FIG. 2 in whichlinescan cameras 20 and 22 view a slab S which has rounded corners C. Asshown the points A and B viewed by the cameras 20 and 22 are points onthe arc of a circle C where the arc of the circle C meets thesubstantially flat top and side surfaces of the slab S. The cornerradius of the curved portion C is a function of rolling practice and ispredictable. Therefore the corner radius of the section C is known andthe points A and B are displaced by a known distance on the arc of thecurved section C. Since the points A and B are displaced by a knowndistance, the location in space of the point A or the point B can beresolved from the images in the linescan cameras 20 and 22.

Another sensing means which can comprise another pair of linescancameras or a further linescan camera in combination with one of thecameras 20 or 22 can inspect points on another corner radius of the slabS so that another point at that corner can be resolved in space toenable a dimension of the slab S to be determined. That dimension couldbe the width W of the slab or the thickness T of the slab depending onwhich other corner of the slab is viewed.

The arrangement illustrated in FIG. 2 could also be used to measure anunknown diameter of a round article by use of the knowledge that twoedges seen by each of the cameras are points on a circle.

In FIG. 3 a further embodiment is shown in which only two cameras areused. One camera 50 views one point A of a pair of points A and Bdisplaced by a known distance and also view one point B' of a secondpair of points A' and B' displaced by a known distance. The secondcamera 52 views the remaining two points B and A'. The location in spaceof one of each pair of points A and B and A' and B' is determined in thesame manner as described above to enable a dimension of the article tobe obtained.

One manner of calculating the location of points A and B will bedescribed with reference to FIG. 4. Using FIG. 4, Xc, Yc, A, B, P, K arecalibration constants determined in the manner set forth in ourAustralian Patent Application No. 41143/85 which is hereby incorporatedby this reference. M is the image location or camera pixel numbercorresponding to an edge seen on the object. From the above applicationwe have the following relationship which locates the position of anobject coordinate in terms of distances x_(d) and y_(d) from theprincipal point of the lens where x_(d) and y_(d) are determined asfollows: ##EQU1## using equation (i) the coordinates of an object point(x_(o), y_(o)) in a cartesian reference frame as above are related asfollows: ##EQU2## θ_(o) is the angle of a ray drawn from principal pointof the camera lens to the object coordinate with respect to the X axis.From the diagram above the following relationships for the object edgecoordinates A and B can be derived with respect to the origin of thecartesian reference frame.

    y.sub.a =tan θ.sub.1 (x.sub.a -X.sub.c1)+Y.sub.c1    (iii)

    y.sub.a -t=tan θ.sub.2 (x.sub.a -X.sub.c2)+Y.sub.c2  (iv)

Combining (iii) and (iv) to eliminate y_(a) gives the location x_(a) interms of calibration constants, plate thickness and measure variables.##EQU3##

Since modification within the spirit and scope of the invention mayreadily be effected by persons skilled within the art, it is to beunderstood that this invention is not limited to the particularembodiment described by way of example hereinabove.

We claim:
 1. A method of determining a dimension of an article, said method comprising the steps of:viewing four points on the article with a sensing means to obtain image positions of said four points, said four points comprising first and second pairs of points being displaced by known relationships; using the known relationships between the first pair of points and the image positions of the first pair of points to determine a location in space of one of said first pair of points; using the known relationships between the second pair of points and the image positions of the second pair of points to determine a location in space of one of said second pair of points; and using the location in space of the said one point of the first pair of points and the location in space of the said one point of the second pair of points to determine a dimension of said article between said one point of the first pair of points and said one point of the second pair of points.
 2. The method of claim 1, wherein a first pair of sensing means each one viewing one of the first pair of points and a second pair of sensing means each one viewing one of the second pair of points.
 3. The method of claim 2, wherein said first pair of sensing means are used with one of said sensing means viewing one of the first pair of points and viewing one of the second pair of points and the other of said pair of sensing means viewing the other of the first pair of points and viewing the other of the second pair of points.
 4. The method of claim 1, wherein the four points comprise points at the edges of a generally rectangular cross-sectioned article and the known relationships comprise the known thickness of the article.
 5. The method of claim 1, wherein the location of the first pair of points (x_(a), Y_(a)); (x_(a), Y_(a-b)) are derived from the following equations wherein, x_(c), Y_(c) are calibration constants, θ is the angle of a ray drawn from principal point of the sensing means to the object coordinate with respect to the X axis, and the subscript 1, 2 denote the respective sensing means;

    y.sub.a =tan θ.sub.1 (x.sub.a -X.sub.c1)+Y.sub.c1    (iii)

    y.sub.a -t=tan θ.sub.2 (x.sub.a -X.sub.c2)+Y.sub.2   (iv) ##EQU4##


6. The method of claim 1, wherein the first and second pair of points are on arcs of circles of curved portions of the article and the known relationships are the radius of the arcs of the circles.
 7. An apparatus for determining a dimension of an article, said apparatus comprising:sensing means for sensing a first and a second pair of points on said article and for providing image positions of said points, the first and second pair of points being displaced by known relationships; and processing means, coupled to said sensing means, for resolving a location in space of one of said first pair of points from said known relationships between the first pair of points and the image positions of the first pair of points, said processing means also for resolving a location in space of one of the points of the second pair of points from the known relationships between the second pair of points and the image positions of the second pair of points, said processing means for determining a distance between one point of the first pair of points and one point of the second pair of points to enable a dimension of the article to be determined. 